To me, the strongest connection between Jesus’s secret teaching and Plato’s is the parable of the sower. Those who argued for Jesus having a secret teaching saw his parables as proof: said Origen, “Jesus explained all things to His own disciples privately; and for this reason the writers of the Gospels have concealed the clear exposition of the parables, because the things signified by them were beyond the power of the nature of words to express.” The parable of the sower is the clearest evidence that Jesus had different teachings for the masses and for his closest followers: exoteric (inside the walls) v. esoteric (outside the walls).
The parable of the sower has very striking similarities to passages from Plato’s Phaedrus and Theages. In the Phaedrus, in the same passages that Socrates says that writing is problematic and higher truths need to be taught orally, he compares teaching to a farmer planting seeds: “Now what about the man who knows what is just, noble, and good? Shall we say that he is less sensible with his seeds than the farmer is with his?… The dialectician chooses a proper soul and plants and sows within it discourse accompanied by knowledge…. Such discourse makes the seed forever immortal and renders the man who has it as happy as any human can be” (276b-277a).
Then Socrates explains that you need to be careful how to speak to people: “You must determine which kind of speech is appropriate to each kind of soul, prepare and arrange your speech accordingly, and offer a complex and elaborate speech to a complex soul and a simple speech to a simple one” (277c).
Just like the parable of the sower, Socrates uses the metaphor of planting seeds and distinguishes between different kinds of speech for different audiences.
In the Theages, Socrates talks about the role his daemon (or genius/guardian spirit) plays in his teaching.
I’ve told you all these things because this spiritual thing [the genius] has absolute power in my dealings with those who associate with me. On the one hand, it opposes many, and it’s impossible for them to be helped by associating with me, so I can’t associate with them. On the other hand, it does not prevent my associating with many others, but it is of no help to them. Those whose association with me the power of the spiritual assists, however—these are the ones you’ve noticed, for they make rapid progress, some are helped in a secure and permanent way, whereas many make wonderful progress as long as they’re with me, but when they go away from me they’re again no different from anyone else (129e-130a).
Andre Dacier, whose French translation of Plato was translated into English in 1701 (the first of Plato’s dialogues in English) argued for a connection to the parable of the sower (and even used the phrase “and takes deep root” for the third type of student). Such seems like a legitimate argument since just like the parable of the sower there are four different types of responses: those who get nothing (like the fowls eating the seed), those who do hear the word but get little benefit (like the stony ground), those who make permanent progress (like the good ground), and those who lose their progress when they go away from Socrates (like the weeds).
Why make such an allusion? I’ll hypothesize about that more in later posts, but at the very least it seems to be an endorsement for Plato’s concept of not writing down the highest truths and for only teaching those truths orally. It also suggests that the secret teaching had something to do with Plato.
 Origen, Commentary on the Gospel of Matthew, 14.12.
 Many don’t consider the Theages to be written by Plato, but it was considered legitimate in antiquity. I’ll talk more about the possible relation between Plato’s spurious treatises and his unwritten doctrine in later posts.
 Andre Dacier, The Works of Plato Abridg’d, 2 vols. (London: A. Bell, 1701), 1:267.